ALTERNATIVE DESIGNS FOR MEGA VOLTAGE MACHINES FOR CANCER TREATMENT IN DEVELOPING COUNTRIES

Transcription

1 ALTERNATIVE DESIGNS FOR MEGA VOLTAGE MACHINES FOR CANCER TREATMENT IN DEVELOPING COUNTRIES C BORRAS Pan American Health Organization, Washington, DC, USA XA H. SVENSSON Imtemational Atomic Energy Agency, Vienna, Austria G.P. HANSON World Health Organization, Geneva, Switzerland Abstract In developing countries radiation therapy is often performed with antiquated cobalt-60 units, the radioactive sources of which are long decayed and, thus, treatments are ineffective. Furthermore the cost involved in the disposal of spent radionuclide sources discourages owners from proper removal and storage, and accidents occur. Although present design of microwave electron linear accelerators provide excellent beam characteristics, developing countries in many locations do not have the infrastructure to maintain such machines. To explore the possibilities of designing, taking advantage of the latest advances in technology, a more elementary electrical teletherapy machine, inexpensive in first cost and maintenance, the Pan American Health Organization / Regional Office of the World Health Organization for the Americas organized in Washington an Advisory Group Consultation on the Design Requirements for Megavoltage X-Ray Machines for Cancer Treatment in Developing Countries, with the collaboration of the World Health Organization, the International Atomic Energy Agency and the United Nations Industrial Development Organization. It was attended by 40 radiation oncologists, physicists, technologists and engineers representatives from radiotherapy equipment manufacturers. After an analysis of the radiotherapy situation world-wide - especially from the viewpoint of maintenance - a consensus was reached on the radiotherapy equipment performance requirements. To meet these requirements, several accelerator designs were considered. Among the most promising new designs were the klystron/linac and the high frequency linear accelerator, the microtron in a radiation head, the high frequency betatron -also in a radiation head-, and DC accelerators. Possible treatment designs, including those of modular nature, were presented. Since it is estimated that by the year barring a dramatic and unforseen cure for cancer - a total of 10,000 machines will be needed to provide treatment for an estimated 10 million new cases per year in developing countries, the impact of such high technology simple machine could be substantial in providing equity and quality for the management of cancer patients. 1. INTRODUCTION According to WHO estimates, currently there are approximately nine million new cancer cases per year, worldwide. This number is expected to increase to about 15 million new cases by the year 2015, with about two-thirds of these cases occurring in developing countries. 93

2 Radiotherapy will, for years to come, be the most important therapy approach for most of these tumors, both for cure and palliation. Surgery is limited in its role due to the advanced stages of the diseases encountered in developing countries and chemotherapy is expensive. In developing countries, the typical incidence is 75 to 150 new cancer patients per,000 population. To serve a current population of 4.4 billion, assuming 4.4 million new cancer cases per year - 50% requiring radiotherapy - and one machine per 500 new cancer cases treated, the current need is a total of 4,400 machines. By the year 2015, barring a dramatic and unforseen cure for cancer, a total of 10,000 machines will be needed to provide treatment for an estimated 10 million new cancer cases per year in developing countries. Presently it is estimated that in developing countries approximately 2,300 megavoltage teletherapy units are installed, primarily cobalt-60. Unfortunately many of these units are antiquated, have received very little maintenance over the years and their radioactive sources are long decayed. Due to economical constraints, source replacement intervals may be up to 10 years, especially in private institutions [1]. Data obtained through the 1992 IAEA/WHO postal dosimetry intercomparison program for high energy radiotherapy units, show that more than 50% of the units tested in Latin American and Caribbean countries would require treatment times of over 4 min to deliver 2 Gy to the tumor. To compensate for the low absorbed dose rates at the treatment distance and still treat a very large number of patients, it has become a common practice to shorten the treatment distance -often without correcting the percentage depth dose tables in clinical use - and to give lower doses than necessary. In no case are doses increased to compensate for the low dose rates. The consequences of these practices are not only inaccurate doses being delivered (the 1992 intercomparison showed errors of more than 38%!), but the fact that treatments are ineffective, fostering the concept that cancer is incurable. Thus the health authorities do not assign proper budgets to radiotherapy services. Furthermore, the cost involved in the disposal of spent radionuclide sources discourages owners from proper removal and storage and accidents like the ones in Ciudad Juarez, Mexico [2], and Goiania, Brazil [3] occur. The industrialized countries have started the process of replacement of cobalt-60 units and most radiation oncology departments in the United States and in Europe have switched to electron accelerators. However, some of these units are very expensive and difficult to maintain and the infrastructure to properly use them is often lacking in developing countries. Thus, for the purpose of improving the availability of radiation therapy for cancer treatment, manufacturers and research laboratories are being encouraged, taking advantage of the latest advances in technology, to consider the design and development of a megavoltage x-ray machine much simpler than present microwave electron accelerators, a machine that could be used both in developed and developing countries. To address this issue the Pan American Health Organization/Regional Office of the World Health Organization for the Americas organized in Washington an Advisory Group Consultation on the Design Requirements for Megavoltage X-Ray Machines for Cancer Treatment in Developing Countries, with the collaboration of the World Health Organization, the International Atomic Energy Agency and the United Nations Industrial Development Organization. It was attended by 40 participants: radiation oncologists, physicists, technologists and engineers representatives from radiotherapy equipment manufacturers. 94

3 After an analysis of the radiotherapy situation world-wide - especially from the viewpoint of maintenance - the Group reached a consensus on performance requirements and proposed various novel accelerator designs. 2. PERFORMANCE REQUIREMENTS To reduce complexity and improve safety a single energy photon unit without electrons is recommended. (It is assumed that there is access to at least superficial x ray machines with energies between and 300 kv for the treatment of tumors up to a depth of 3 cm.) - Treatment time (average 2 fields/patient): min/patient - Patients treated/8 hours day: Dose rate at isocenter (depth of dose maximum): minimum 0.8 Gy/min recommended 2-3 Gy/min 2.1 Mechanical data - Isocentric design recommended - Isocentric height above floor level <130 cm, preferably 115 cm - Isocentric clearance (with all devices) >35 cm - Source-isocenter-distance >80 cm, preferably cm The floor surface should preferably be flat (no pit). (A small depression is acceptable). Collimator jaw and distance indication: mechanical or electrical with mechanical backup. 2.2 Couch motions and radiation field size - Isocentric, rotation is preferred. - Angle of rotation ±90 - Lateral range ±20 cm - Vertical range 70 cm below isocenter preferred. - Field sizes up to 42 X 42 cm 2 at the patient surface of 25 cm thick patient, should be available from above with lowered table (Maximum Field Size at isocenter >30x30cm 2 ) 2.3 Radiation Beam Quality The beam quality is defined in a parallel opposed 1 beam configuration for a 10 x 10 cm 2 field size and a patient thickness of 25 cm using equal beam weights. In this configuration the following should hold: - Depth of superficial 90% isodose 2 <5 mm - Hot spot relative a central target 3 =115% - Penumbra width < 1 cm, and preferably < 8 mm - Uniformity over 80% of field (IEC) ±3% It is highly desirable that the hot spot is not greater than 110% relative to a central target. (See Table I for Dose nuxmum/dose»* for various beam energies). 95

4 About 65% of all radiation therapy is with two opposing fields and there is a strong preference for the higher energy (6 MV) instead of using more than two fields with lower energy (e.g., 2.5 MV equivalent to cobalt-60). (See Table II). To treat superficial lymph nodes. To avoid fibrosis. Table I DM/DA RATIOS FOR VARIOUS BEAM ENERGIES Patient Thickness Machine Depth(mm) SSD 20 cm 25 cm DM D90 (cm) DM/DA DM/DA Co % 127% Co MV MV MV MV MV Table II 115% DM/DA REQUIREMENT 25 cm thick patient 1:1 Parallel Opposed Fields Photon Energy Co-60, 4 MV 5MV 6MV 8MV No SSD treatments OK SAD treatments OK Better, DM/DA = 110% Electron Beam Current 6MV 4MV (3 Gy/ndn, an SAD) ^A 200 na 96

5 2.4 Devices - to be available for radiotherapy treatment - light indication for field size with central axis indication - distance indication with mechanical backup - isocentric indication with mechanical backup - wedges 15, 30, 45, 60" light field preferably visible after insertion, orientation and wedge angle interlock. - shadow tray(s) for standard and customized beam blocks. - possibility to take megavoltage port films 2.5 Safety - Compliance with FAO/IAEA/ILO/OECD-NEA/PAHO/WHO International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources, as well as national and local safety regulations. 2.6 Quality and Maintenance - Uptime >95% - Service interventions* < I/month - Preventative maintenance < 4/year - Self diagnosis is recommended - Component potential failure status read-out recommended * Because clinical treatment cannot continue 2.7 Serviceability/Reliability Specifications Preventative Maintenance There shall be specified intervals for Preventative Maintenance. The integrity of the machine shall not be guaranteed if the manufacturer's schedule is not followed. These specifications consume other maintenance than the regular (daily/weekly) QA checks on the equipment Failures requiring intervention These shall be classified according to the level of skill required to rectify the fault. With good education programs, some of these interventions may be handled "in house". a) First Line - This is a local engineer trained by the manufacturer or the manufacturers local organization as appropriate. Diagnose problem to unit level using standard fault finding practices. In most cases isolate fault to Printed Circuit Board level. Solve 90% of problems. b) Second Line - Manufacturer's engineers usually at a regional rather than hospital levels. System oriented - able to solve 80% of the remaining 10% of problems, i.e. 98%. c) Third Line - Manufacturer's engineer of a senior level usually at the Head office. 97

6 2.7.3 Meantime between failures Failures capable of first line repair > 3 months Failures capable of second line repair > 1 year Failures capable of third line repair > 10 years Target Planned Maintenance Schedules 1 day required every 3 months 3 days required every year Spare parts A stocking strategy should be defined, e.g.: First line repairs Second line repairs Third line repairs on site regional manufacturer 3. ALTERNATIVE NEW APPROACHES The various accelerator technologies considered were subjectively ranked A, B, or C depending upon their practicality and likely ability to meet the above requirements. Ranking Definition A Technology is most likely to meet the requirements and merits further exploration B C Technology is probably relevant Technology is not likely to meet the requirements, and is not recommended for exploration at this time but should be retained for future reference. 3.1 Category A Microwave linac Microwave power source - linac Microtron in radiation head 2 Beam klystron/linac accelerator Modular design linac 3.2 Category B 98 Betatron in radiation head Rhodotron Continuous wave rf linac or microtron DC Accelerators

7 Cascade voltage multiplier Laddertron or Pelletron charged electrostatic accelerator Nested high voltage generator Transformer coupled high voltage generator 3.3 Category C Plasma wave accelerator Induction linac Interlaced accelerator structure Superconductig linac using superconductor or beryllium coated cavities Small synchrotron Multiple low power magnetron Accelerator activated short lived isotope 4. BRIEF DESCRIPTION OF PREFERRED UNITS 4.1 Hie Klystron/Linac and High Frequency Linear Accelerator Each of these offered the possibility of a compact 6 MV gantry mounted accelerator. The klystron/linac and the integrated klystron/accelerator waveguide system do not require a particular resonance frequency. Higher frequency accelerators of either the linac or microtron type would be less massive. 4.2 "Mini-Microtron" This design allows the possibility of a compact (35-40 cm diameter) 6 MV microton to be mounted in the gantry at the top of the radiation head. The small size of the accelerator can be achieved either by use of a high frequency or by a high energy gain per turn. It was considered that an integrated magnetron/cavity design might be advantageous. The microtron layout allows the field flattener to be in the fringing magnetic field thus reducing secondary electron emission. A photon beam spoiler may be employed for control of dose build-up. The accelerator should fit in a 80 cm SAD gantry, possibly even a cm SAD gantry. 4.3 "Mini-Betatron" In this design a small, 25 cm diameter donut betatron mounted in the gantry at the top of the head provides a compact 6 MV machine. A high frequency (possibly 10 Khz) is required to provide adequate output. A DC bias can be used to double the energy gain. 4.4 DC Accelerators A relatively compact gantry mounted accelerator can be achieved using DC accelerator principles. This machine was considered to be potentially highly reliable. However, the 2-3 MV energy achievable was generally considered lower than required by most users. Heavy filtration of this beam would allow beam characteristics similar to a conventional 3 or 4 MV accelerator. The adequacy of the beam current obtainable was questioned. 99

8 4.5 Classical linacs (rf) Commercial 4-6 MV linacs of compact design are widely available, though general concerns of cost and reliability were expressed. 5. CONCLUSIONS The current manufacturers of electron linear accelerators and microtrons should be encouraged to design and prototype a super-reliable 6 MV x-ray system subject to the established performance specifications. This encouragement should come from accelerator designers who might cooperate with the manufacturers as well as representatives of the developing nations who can best make the case for their needs. REFERENCES [1] BORRAS, C. "Cobalt Therapy Source Replacement and Disposal Problems in Latin America", Proceedings World Congress on Medical Physics and Biomedical Engineering. Rio de Janeiro, Brazil (1994). [2] SECRETARIA DE ENERGIA, MINAS E INDUSTRIA PARAESTATAL, "Acci-dente por Contaminaci6n con Cobalto-60". Mexico (1984). [3] INTERNATIONAL ATOMIC ENERGY AGENCY, "The Radiological Accident in Goiania, Brazil", Vienna, Austria (1988). [4] PAN AMERICAN HEALTH ORGANIZATION, WORLD HEALTH ORGANI- ZATION, INTERNATIONAL ATOMIC ENERGY AGENCY, UNITED NATIONS INDUSTRIAL DEVELOPMENT ORGANIZATION, "Design Requirements for Megavoltage X-Ray Machines for Cancer Treatment in Developing Countries". In preparation.